Classification and Identification of Perfume Scents by an Enantioselective Colorimetric Sensor Array of Chiral Metal-Organic-Framework-Based Fabry-Pérot Films.

Many odors, like perfumes, are complex mixtures of chiral and achiral molecules where the cost-efficient (enantio-)selective sensing represents a major technical challenge. Here, we present a colorimetric sensor array of surface-mounted metal-organic-framework (SURMOF) films in Fabry-Pérot (FP) cavities. The optical properties of the FP-SURMOF films with different chiral and achiral structures are affected by the (enantio-)selective adsorption of the analytes in the SURMOF pores, resulting in different responses to the analyte molecules. The read-out of the sensor array is performed by the digital camera of a common smartphone, where the RGB values are determined. By analyzing the sensor array data with simple machine learning algorithms, the analytes are discriminated. After demonstrating the enantioselective response for a pair of pure chiral odor molecules, we apply the sensor array to detect and discriminate a large number (16) of common commercial perfumes and eau de toilettes. While our untrained human nose is not able to discriminate all perfumes, the presented colorimetric sensor array can classify all perfumes with great classification accuracy. Moreover, the sensor array was used to identify unlabeled samples correctly. We foresee such an FP-chiral-SURMOF-based sensor array as a powerful approach toward inexpensive selective odors sensing applications.

Optimized Detection of Volatile Organic Compounds Utilizing Durable and Selective Arrays of Tailored UiO-66-X SURMOF Sensors.

Metal-organic frameworks (MOFs), with their well-defined and highly flexible nanoporous architectures, provide a material platform ideal for fabricating sensors. We demonstrate that the efficacy and specificity of detecting and differentiating volatile organic compounds (VOCs) can be significantly enhanced using a range of slightly varied MOFs. These variations are obtained via postsynthetic modification (PSM) of a primary framework. We alter the original MOF's guest adsorption affinities by incorporating functional groups into the MOF linkers, which yields subtle changes in responses. These responses are subsequently evaluated by using machine learning (ML) techniques. Under severe conditions, such as high humidity and acidic environments, sensor stability and lifespan are of utmost importance. The UiO-66-X MOFs demonstrate the necessary durability in acidic, neutral, and basic environments with pH values ranging from 2 to 11, thus surpassing most other similar materials. The UiO-66-NH2 thin films were deposited on quartz-crystal microbalance (QCM) sensors in a high-temperature QCM liquid cell using a layer-by-layer pump method. Three different, highly stable surface-anchored MOFs (SURMOFs) of UiO-66-X obtained via the PSM approach (X: NH2, Cl, and N3) were employed to fabricate arrays suitable for electronic nose applications. These fabricated sensors were tested for their capability to distinguish between eight VOCs. Data from the sensor array were processed using three distinct ML techniques: linear discriminant (LDA), nearest neighbor (k-NN), and neural network analysis methods. The discrimination accuracies achieved were nearly 100% at high concentrations and over 95% at lower concentrations (50-100 ppm).

Photoswitchable Radical State in Nanoporous Metal-Organic Framework Films with Embedded Hexaarylbiimidazole.

Photoresponsive materials enable dynamic remote control of their fundamental properties. The incorporation of photochromic molecules in nanoporous metal-organic frameworks (MOFs) provides a unique opportunity to tailor the material properties, including the interaction between the MOF host and guest molecules in the pores. Here, a MOF film of type HKUST-1 with embedded hexaarylbiimidazole (HABI), undergoing reversible light-induced reactions between the stable dimer state and the metastable radical state, is presented. The switching between the dimer and radical form is shown by infrared, UV-vis, and electron paramagnetic resonance (EPR) spectroscopy. By transient uptake experiments with ethanol and methanol as probe molecules, we show that the dimer-radical switching impacts the host-guest interaction and, in particular, modifies the uptake amount and diffusion coefficient of the guest molecules. For ethanol, the diffusion slows down by 75%. This research presents the first MOF material with photoswitchable (meta)stable dimer and radical molecules, and it contributes to the advancement of photoresponsive nanoporous materials.

On-off conduction photoswitching in modelled spiropyran-based metal-organic frameworks.

Materials with photoswitchable electronic properties and conductance values that can be reversibly changed over many orders of magnitude are highly desirable. Metal-organic framework (MOF) films functionalized with photoresponsive spiropyran molecules demonstrated the general possibility to switch the conduction by light with potentially large on-off-ratios. However, the fabrication of MOF materials in a trial-and-error approach is cumbersome and would benefit significantly from in silico molecular design. Based on the previous proof-of-principle investigation, here, we design photoswitchable MOFs which incorporate spiropyran photoswitches at controlled positions with defined intermolecular distances and orientations. Using multiscale modelling and automated workflow protocols, four MOF candidates are characterized and their potential for photoswitching the conductivity is explored. Using ab initio calculations of the electronic coupling between the molecules in the MOF, we show that lattice distances and vibrational flexibility tremendously modulate the possible conduction photoswitching between spiropyran- and merocyanine-based MOFs upon light absorption, resulting in average on-off ratios higher than 530 and 4200 for p- and n-conduction switching, respectively. Further functionalization of the photoswitches with electron-donating/-withdrawing groups is demonstrated to shift the energy levels of the frontier orbitals, permitting a guided design of new spiropyran-based photoswitches towards controlled modification between electron and hole conduction in a MOF.

Optical sensor array of chiral MOF-based Fabry-Pérot films for enantioselective odor sensing.

An optical sensor array based on photonic Fabry-Pérot films of surface-mounted metal-organic-frameworks (SURMOFs) with different homochiral structures is presented. It is used to detect and enantioselectively discriminant 3 pairs of chiral odor molecules, either pure or in binary mixtures.

Substrate-Bound Diarylethene-Based Anisotropic Metal-Organic Framework Films as Photoactuators with a Directed Response.

Molecular machines and responsive materials open a plethora of new opportunities in nanotechnology. We present an oriented crystalline array of diarylethene (DAE)-based photoactuators, arranged in a way to yield an anisotropic response. The DAE units are assembled, together with a secondary linker, into a monolithic surface-mounted metal-organic framework (SURMOF) film. By Infrared (IR) and UV/Vis spectroscopy as well as by synchrotron X-ray diffraction, we show that the light-induced extension changes of the molecular DAE linkers multiply to yield mesoscopic and anisotropic length changes. Due to the special architecture and substrate-bonding of the SURMOF, these length changes are transferred to the macroscopic scale, leading to the bending of a cantilever and performing work. This research shows the potential of assembling light-powered molecules into SURMOFs to yield photoactuators with a directed response, presenting a path to advanced actuators.

Twisting of Porphyrin by Assembly in a Metal-Organic Framework yielding Chiral Photoconducting Films for Circularly-Polarized-Light Detection.

While materials based on organic molecules usually have either superior optoelectronic or superior chiral properties, the combination of both is scarce. Here, a crystalline chiroptical film based on porphyrin with homochiral side groups is presented. While the dissolved molecule has a planar, thus, achiral porphyrin core, upon assembly in a metal-organic framework (MOF) film, the porphyrin core is twisted and chiral. The close packing and the crystalline order of the porphyrin cores in the MOF film also results in excellent optoelectronic properties. By exciting the Soret band of porphyrin, efficient photoconduction with a high On-Off-ratio is realized. More important, handedness-dependent circularly-polarized-light photoconduction with a dissymmetry factor g of 4.3×10-4 is obtained. We foresee the combination of such assembly-induced chirality with the rich porphyrin chemistry will enable a plethora of organic materials with exceptional chiral and optoelectronic properties.

Nanoporous Films with Oriented Arrays of Molecular Motors for Photoswitching the Guest Adsorption and Diffusion.

Molecular motors are fascinating nanomachines. However, constructing smart materials from such functional molecules presents a severe challenge in material science. Here, we present a bottom-up layer-by-layer assembly of oriented overcrowded-alkene molecular motors forming a crystalline metal-organic framework thin film. While all stator parts of the overcrowded-alkene motors are oriented perpendicular to the substrate, the rotors point into the pores, which are large enough allowing for the light-induced molecular rotation. Taking advantage of the thin film's transparency, the motor rotation and its activation energy are determined by UV/Vis spectroscopy. As shown by gravimetric uptake experiments, molecular motors in crystalline porous materials are used, for the first time, to control the adsorption and diffusion properties of guest molecules in the pores, here, by switching with light between the (meta-)stable states. The work demonstrates the potential of designed materials with molecular motors and indicates a path for the future development of smart materials.

Optical and chemical control of the wettability of nanoporous photoswitchable films.

Wettability is a central surface property of functional thin films. Here, we present a nanoporous film made of an azobenzene-containing metal-organic framework material where the wettability is controlled by photoswitching of the fluorinated azobenzene moieties and by reversible incorporation of guest molecules with different polarities in the pores. Using both, the optical and the chemical stimuli, the water contact angle was modified over a wide range, from 23° to 97°.

Nanoporous Metal-Organic Framework Thin Films with Embedded Fulgide for Light-Modulated Guest Adsorption and Diffusion.

Smart and photoresponsive materials and thin films allow the dynamic remote control of their central properties. By incorporation of photochromic molecules in nanoporous metal-organic frameworks (MOFs), the interaction between the MOF host and the guest molecules in the pores can be modified. Here, a MOF film of type UiO-68 is presented in which the photoswitchable feature is added by embedment of photochromic fulgide molecules of type Aberchrome 670 in the pores. The photoisomerization in the pores is explored by UV-vis and infrared spectroscopy, and the transient uptake of toluene and methanol probe molecules is explored using a quartz crystal microbalance. For the first time, a fulgide-based nanoporous material is used to remote-control the adsorption and diffusion properties. We find that the toluene uptake amount can be increased by 37% and the toluene diffusion coefficient can be increased by 40% when reversibly photoswitching the embedded fulgide from its E-form to C-form. In this way, this study aims to contribute to the field of light-responsive nanoporous materials and thus expands the range of smart coatings.

Controlling the Mobility of Ionic Liquids in the Nanopores of MOFs by Adjusting the Pore Size: From Conduction Collapse by Mutual Pore Blocking to Unhindered Ion Transport.

Ionic liquids (ILs) in nanoporous confinement are the core of many supercapacitors and batteries, where the mobility of the nanoconfined ILs is crucial. Here, by combining experiments based on impedance spectroscopy with molecular dynamics simulations, the mobility of a prototype IL in the nanopores of an isoreticular metal-organic framework (MOF)-series with different pore sizes is explored, where an external electric field is applied. It has been found that the conduction behavior changes tremendously depend on the pore size. For small-pore apertures, the IL cations and anions cannot pass the pore window simultaneously, causing the ions to mutually block the pores. This results in a strong concentration dependence of the ionic conduction, where the conduction drops by two orders of magnitude when filling the pores. For large-pore MOFs, the mutual hindrance of the ions in the pores is small, causing only a small concentration dependence. The cutoff between the large-pore and small-pore behavior is approximately the size of a cation-anion-dimer and increasing the pore diameter by only 0.2 nm changes the conduction behavior fundamentally. This study shows that the pore aperture size has a substantial effect on the mobility of ions in nanoporous confinement and has to be carefully optimized for realizing highly-mobile nanoconfined ILs.

VOC Mixture Sensing with a MOF Film Sensor Array: Detection and Discrimination of Xylene Isomers and Their Ternary Blends.

Detection and recognition of volatile organic compounds (VOCs) are crucial in many applications. While pure VOCs can be detected by various sensors, the discrimination of VOCs in mixtures, especially of similar molecules, is hindered by cross-sensitivities. Isomer identification in mixtures is even harder. Metal-organic frameworks (MOFs) with their well-defined, nanoporous, and versatile structures have the potential to improve the VOC sensing performance by tailoring the adsorption affinities. Here, we detect and identify ternary xylene isomer mixtures by using an array of six gravimetric, quartz crystal microbalance (QCM)-based sensors coated with selected MOF films with different isomer affinities. We use classical molecular simulations to provide insights into the sensing mechanism. In addition to the attractive interaction between the analytes and the MOF film, the isomer discrimination is caused by the rigid crystalline framework sterically controlling the access of the isomers to different adsorption sites in the MOFs. The sensor array has a very low limit of detection of 1 ppm for each pure isomer and allows the isomer discrimination in mixtures. At 100 ppm, 16 different ternary o-p-m-xylene mixtures were identified with high classification accuracy (96.5%). This work shows the unprecedented performance of MOF-sensor arrays, also referred to as MOF-electronic nose (MOF-e-nose), for sensing VOC mixtures. Based on the study, guidelines for detecting and discriminating complex mixtures of volatile molecules are also provided.

Mass transfer of toluene in a series of metal-organic frameworks: molecular clusters inside the nanopores cause slow and step-like release.

The mass transfer of the guest molecules in the pores is fundamental for the application of nanoporous materials like metal-organic frameworks, MOFs. In the present work, we explore the uptake and release of toluene in a series of Zr-based MOFs with different pore sizes. We find that intermolecular guest-guest interaction, sterically controlled by the pore size, has a substantial impact on the release kinetics. While the adsorption is rather fast, the desorption process is many orders of magnitude slower. Depending on the pore size, molecular clusters form, here (most likely) toluene dimers, which are rather stable and their break-up is rate-limiting during the desorption process. This results in a step-like desorption kinetics, deviating from the plain Fickian-diffusion-controlled release. Temperature-dependent experiments show that the minimum and maximum of the release rates are obtained at the same toluene loadings, independent of the temperature. Moreover, the activation energy for the release coincides with the binding energy of a toluene dimer. The work shows the importance of intermolecular guest-guest interaction, controlled by the MOF-nanoconfinement, for the uptake and release from nanoporous materials.

Test-Retest reliability of the internal shoulder rotator muscles' stretch reflex in healthy men.

Until now the reproducibility of the short latency stretch reflex of the internal rotator muscles of the glenohumeral joint has not been identified. Twenty-three healthy male participants performed three sets of external shoulder rotation stretches with various pre-activation levels on two different dates of measurement to assess test-retest reliability. All stretches were applied with a dynamometer acceleration of 104°/s2 and a velocity of 150°/s. Electromyographical response was measured via surface EMG. Reflex latencies showed a pre-activation effect (ƞ2 = 0,355). ICC ranged from 0,735 to 0,909 indicating an overall "good" relative reliability. SRD 95% lay between ±7,0 to ±12,3 ms. The reflex gain showed overall poor test-retest reproducibility. The chosen methodological approach presented a suitable test protocol for shoulder muscles stretch reflex latency evaluation. A proof-of-concept study to validate the presented methodical approach in shoulder involvement including subjects with clinically relevant conditions is recommended.

Sensing Molecules with Metal-Organic Framework Functionalized Graphene Transistors.

Graphene is inherently sensitive to vicinal dielectrics and local charge distributions, a property that can be probed by the position of the Dirac point in graphene field-effect transistors. Exploiting this as a useful sensing principle requires selectivity; however, graphene itself exhibits no molecule-specific interaction. Complementarily, metal-organic frameworks can be tailored to selective adsorption of specific molecular species. Here, a selective ethanol sensor is demonstrated by growing a surface-mounted metal-organic framework (SURMOF) directly onto graphene field-effect transistors (GFETs). Unprecedented shifts of the Dirac point, as large as 15 V, are observed when the SURMOF/GFET is exposed to ethanol, while a vanishingly small response is observed for isopropanol, methanol, and other constituents of the air, including water. The synthesis and conditioning of the hybrid materials sensor with its functional characteristics are described and a model is proposed to explain the origin, magnitude, and direction of the Dirac point voltage shift. Tailoring multiple SURMOFs to adsorb specific gases on an array of such devices thus generates a versatile, selective, and highly sensitive platform for sensing applications.

Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit.

The conduction of protons and other ions in nanoporous materials, such as metal-organic frameworks (MOFs), is intensively explored with the aim of enhancing the performance of energy-related electrochemical systems. The ionic conductivity, as a key property of the material, is typically determined by using electrochemical impedance spectroscopy (EIS) in connection with a suitable equivalent circuit. Often, equivalent circuits are used where the physical meaning of each component is debatable. Here, we present an equivalent circuit for the ionic conduction of electrolytes in nanoporous, nonconducting materials between inert and impermeable electrodes without faradaic electrode reactions. We show the equivalent circuit perfectly describes the impedance spectra measured for the ion conduction in MOFs in the form of powders pressed into pellets as well as for MOF thin films. This is demonstrated for the ionic conduction of an aprotic ionic liquid, and of various protic solvents in different MOF structures. Due to the clear physical meaning of each element of the equivalent circuit, further insights into the electrical double layer forming at the MOF-electrode interface can be obtained. As a result, EIS combined with the appropriate reference circuit allows us to make statements of the quality of the MOF-substrate interface of different MOF-film samples.

Programmed Molecular Assembly of Abrupt Crystalline Organic/Organic Heterointerfaces Yielding Metal-Organic Framework Diodes with Large On-Off Ratios.

Structurally well-defined, crystalline organic/organic heterojunctions between C60- and anthracene-based semiconductors are realized via layer-by-layer deposition of metal-organic framework, MOF, thin films. As demonstrated by X-ray diffraction, perfect epitaxy is achieved by adjusting the lattice constants of the two different MOFs. Deposition of top electrodes allows to fabricate p-n as well as n-p devices. Measurements of the electrical properties reveal the presence of high-performance diodes, with a current on/off ratio of up to 6 orders of magnitude and an ideality factor close to unity. The crystalline nature of the abrupt organic/organic heterojunction provides the basis for a rational, simulation-based optimization and tailoring of such organic semiconductor interfaces.

Chirality Remote Control in Nanoporous Materials by Circularly Polarized Light.

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal-organic framework (MOF). The azobenzene undergoes trans-to-cis-photoisomerization upon irradiation with green light and reverts back to trans upon violet light. While each moiety in cis conformation is chiral, we show the trans isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, R- and S-cis as well as R- and S-trans, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites R-cis and R-trans enantiomers, producing a MOF with enriched S-enantiomers, and vice versa. The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, trans and cis, are stabilized in nonplanar, chiral conformations in the MOF, the trans isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.

Structural and Dynamic Insights into the Conduction of Lithium-Ionic-Liquid Mixtures in Nanoporous Metal-Organic Frameworks as Solid-State Electrolytes.

Metal-organic framework (MOF)-based separators in Li-ion batteries (LIBs) have the potential to improve the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials acting as (quasi) solid-state electrolytes are crucial for the battery power output. Here, we investigate the mobility of a Li-based IL in MOF nanopores and unveil the details of the conduction mechanism by molecular dynamics (MD) simulations. A complex conductivity depending on the Li-IL loading and on the IL composition is observed. Most importantly, the presence of Li prevents the collapse of the conductivity at high IL loadings. The fully atomistic MD simulations including guest-guest and guest-host interactions elucidate the competing mechanisms: Li follows a Grotthuss-like conduction mechanism with large mobility. While at small pore fillings, the Li conduction is limited by the large distance between the anions facilitating the Grotthuss-like conduction; the conduction at high pore fillings is governed by field-induced concentration inhomogeneities. Because of the small MOF pore windows, which hinders the simultaneous passage of the large IL cations and anions in opposite directions, the IL shows field-induced MOF pore blocking and ion bunching. The regions of low anion concentration and high cation concentration represent barriers for Li, decreasing its mobility. In comparison to Li-free IL, the IL bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, resulting in a tremendously increased conductivity at maximum pore filling. The exploitation of this mechanism may enhance the development of advanced batteries based on IL and nanoporous separators.

Photoswitchable Metal-Organic Framework Thin Films: From Spectroscopy to Remote-Controllable Membrane Separation and Switchable Conduction.

The preparation of functional materials from photoswitchable molecules where the molecular changes multiply to macroscopic effects presents a great challenge in material science. An attractive approach is the incorporation of the photoswitches in nanoporous, crystalline metal-organic frameworks, MOFs, often showing remote-controllable chemical and physical properties. Because of the short light-penetration depth, thin MOF films are particularly interesting, allowing the entire illumination of the material. In the present progress report, we review and discuss the status of photoswitchable MOF films. These films may serve as model systems for quantifying the isomer switching yield by infrared and UV-vis spectroscopy as well as for uptake experiments exploring the switching effects on the host-guest interaction, especially on guest adsorption and diffusion. In addition, the straightforward device integration facilitates various experiments. In this way, unique features were demonstrated, such as photoswitchable membrane separation with continuously tunable selectivity, light-switchable proton conductivity of the guests in the pores, and remote-controllable electronic conduction.